Isostripper with normal butane removal in an alkylation system

ABSTRACT

AN ISOSTRIPPER IS PROVIDED IN AN ALKYLATION FRACTIONATION SYSTEM IN WHICH A PORTION OF THE ISOBUTENE IS INITIALLY DISCHARGED FROM A DEPROPANIZER, AND A PORTION OF THE ALKYLATION HYDROCARBON FEEDSTREAM IS PASSED TO THE ISOSTRIPPER AT A LOCATION ABOVE THE ENTRY OF THE DEPROPANIZED STREAM. A NORMAL BUTANE VAPOR STRAM IS REMOVED FROM THE MIDDLE PORTION OF THE ISOSTRIPPER ADJACENT A HEATING ZONE THEREIN. THE ISOSTRIPPER REQUIREMENT IS THUS LOWER AND IT ALSO OPERATES IN A MORE UNIFORM AND CONSTANT MANNER.   D R A W I N G

March 7, 1972 c. c. CHAPMAN 3,647,905

ISOSTRIPPER WITH NORMAL BUTANE REMOVAL IN AN ALKYLATION SYSTEM Filed Jan. 29, 1970 INVENTOR. C. C. CH APMAN BY Q A T TORNEVS United States Patent 3,647,905 ISOSTRIPPER WITH NORMAL BUTANE REMOVAL IN AN ALKYLATION SYSTEM Charles C. Chapman, Bartlesville, Okla, assignor to Phillips Petroleum Company Filed Jan. 29, 1970, Ser. No. 6,658 Int. Cl. B01d 3/00; C07c 3/50; Cg 7/00 US. Cl. 260683.4 2 Claims ABSTRACT OF THE DISCLOSURE An isostripper is provided in an alkylation fractionation system in which a portion ofthe isobutane is initially discharged from a depropanizer, and a portion of the alkylation hydrocarbon feedstream is passed to the isostripper at a location above the entry of the depropanized stream. A normal butane vapor stream is removed from the middle portion of the isostripper adjacent a heating zone therein. The isostripper requirement is thus lower and it also operates in a more uniform and constant manner.

This invention resides in an improved isostripper system. In another aspect, this invention resides in an improved method of charging feedstreams to an isostripper of an alkylation fractionation system.

Heretofore, isostripper systems were generally charged with a single hydrocarbon feedstream. The entry of a single feedstream having a substantially constant quantity and composition often overloaded the isostripper. Such vapor load, isobutane and lighter, on the isostripper required that a very large isostripper be used in the conventional operation.

It is therefore an object of this invention to provide an improved isostripper system. Another object of this invention is to provide an improved method of charging an isostripper of an alkylation fractionation system. Yet another object of this invention is to provide a method for operating an isostripper of an alkylation fractionation system whereby a product of substantially more uniform gradation is maintained through said isostripper. Other aspects, and advantages of the present invention will become apparent from a study of the disclosure, the appended claims, and the drawing.

The drawing is a diagrammatic view in partial section of a depropanizer, an isostripper, and associated equipment.

Referring to the drawing, a hydrocarbon feedstream is flowing through line 2 from an alkylation settler (not shown). The feedstream is split into first and second feedstreams 4, 6. The first feedstream 4 flows into a chamber 8 of a depropanizer 10. The depropanizer 10 has upper, middle, and lower portions 12, 14, 16. A bottom product stream 18 flows from the lower portion 16 and reboiling means such as indicated by an indirect heater 20 is positioned within the chamber 8 of the lower portion 16. External heating for reboiling, of course, can be used. The first feedstream 4 enters the chamber 8 at an upper end of the middle portion 14 of the depropanizer 10, and a liquid stream 22 comprising isobutane discharges from the depropanizer 10 at a location higher in elevation than the first feedstream 4 entering the depropanizer 10 and is positioned in the lower portion of the upper portion 12 of the depropanizer 10. A reflux line 24 discharges liquid into the upper portion 12 of the depropanizer It and a top discharge line 26 discharges vapor rich in propane from the upper portion 12 of the depropanizer 10. Fluid for reflux line 24 is obtained by condensing a portion of the fluid stream flowing through line 26. The amount of net propane removed by way of 26 is sufiicient to prevent 3,647,905 Patented Mar. 7, 1972 buildup of propane within the alkylation system which would be returned to the alkylation system. with the isobutane recycle 22.

The second feedstream 6 flows into a chamber 28 of an isostripper 30. The-isostripper 30 has upper, middle, and lower portions 32, 34, 36. The second feedstream 6 flows into the upper portion 32 of the isostripper 30. The depropanizer bottom product stream 18 flows into the upper portion 32 of the isostripper 30 at a location lower in elevation than the second stream 6 entering from the depropanizer 10 and spaced a preselected vertical distance from the second stream 6 with said distance being relative to the change of fluid properties through the isostripper column 30. It is preferred that the second feedstream 6 enter the isostripper 30 at a location above the uppermost tray therein as a reflux liquid, and the bottom product stream 18 enter the isostripper 30 adjacent the position at which the fluid properties within isostripper 30 are most nearly like the fluid properties of the fluid forming the bottom product stream 18. That location can be determined by one skilled in the art from the feedstream 18 composition and the compositions on the various trays in the isostripper. Such data can be determined from column sampling and testing. By so separating the streams charged to the isostripper as stream 6 and stream 18, the latter having had removed therefrom a portion of the propane and of the isobutane by way of conduits 26 and 22, respectively, a product of more uniform gradation is maintained throughout the isostripper, and a smaller isostripper can be used. This condition allows the isostripper system to function in a more economical and eflicient manner and not to be and is not upset by the charging fluids.

A heating zone 38 is provided in the middle portion 34 of the isostripper, and a vapor stream 40 comprising normal butane is removed from a locus adjacent the heating zone 38. A stream heater 42, for example, maintains the desired temperature of the heating zone 38. A reboiler 44 and a product discharge stream 46 are positioned within the lower portion 36 of the isostripper 30. An isobutane stream 48 is discharged from the isostripper 30 for recycle to the alkylation system. An example of an alkylation fractionation system within which the process of this invention can be used is found in US. Patent No. 3,253,054.

In the operation of this invention, a portion of the feedstream is charged to the depropanizer to remove, as vapor, a portion of the propane made in the alkylation operation. A liquid isobutane stream is removed from the depropanizer above the feed locus. Removal of this feedstream decreases the vapor load of the isostripper. The system can therefore utilize an isostripper of reduced size while maintaining the size of the depropanizer owing to the fact that isobutane is recovered from said depropanizer as a liquid. The amount of charge to the depropanizer 12 Via line 4 is an amount suflicient to remove a volume of propane as large or larger than that volume of propane that is in the feed and/or is produced in the previous alkylation step for example.

An example of the operation of the isostripper system of this invention is as follows:

EXAMPLE Depropanizer 10:

Pressure, p.s.i.a 300 Top temperature F. 128 Bottom temperature, F. 253 Isostripper 30:

Pressure, p.s.i.a 132 Top temperature, F. 144 Bottom temperature, F. 336

Stream No 4 22 18 6 48 40 46 1 Component:

Hydrogen fluoride 477 5 583 Ethane 35 8 43 Propane. 7, 008 3, 406 21 8, 565 Isobutane 61, 196 45, 519 15, 451 74, 776 Normal butane- 4, 087 2, 223 1, 844 4, 993 Isopentane plus 8, 913 280 8, 536 10, 883

Total 81, 716 51, 441 25, 853 99, 843 105, 712 986 18, 908

1 N et.

Nora-Quantities are barrels per day on a liquid ba is.

When operating in accordance with the invention, the isostripper requirement is only about 70 percent of that required in conventional operations. Note, about 45,519 barrels per day of isobutane are removed as liquid, not vapor, from the depropanizer. This leaves only about 90,225 barrels per day of isobutane to be removed as vapor from the isostripper, instead of the conventional quantity of about 135,746 barrels per day removed as vapor from the isostripper in a conventional system. Equipment of smaller size can therefore be utilized and the isostripper system will operate in a more uniform constant manner.

Other modifications and alterations of this invention will become apparent to those skilled in the art from the foregoing discussion and accompanying drawing and it should be understood that this invention is not to be unduly limited thereto.

What is claimed is:

1. In an improved method of charging an isostripper of an alkylation fractionation system comprising a hydrocarbon feedstream, a depropanizer having upper, middle, and lower portions, with a heater within the lower portion of the depropanizer and a bottom product stream flowing from the lower portion of the depropanizer, and an isostripper having upper, middle, and lower portions, the improvement comprising:

separating the hydrocarbon feedstream into first and second hydrocarbon feedstreams;

passing the first hydrocarbon feedstream into the depropanizer;

passing the second hydrocarbon feedstream into the upper portion of the isostripper;

passing the depropanizer bottom product stream into the isostripper at a location lower in elevation than the location at which said second hydrocarbon feedstream enters the isostripper;

discharging a liquid stream from the depropanizer at a location higher in elevation than said first hydrocarbon feedstream entering the depropanizer, said liquid stream comprising isobutane; and

removing a vapor stream from the middle portion of the isostripper at a location adjacent a heating zone contained therein, said vapor stream comprising normal butane.

2. A method, as set forth in claim 1, wherein the depropanized bottom product stream is passed into the isostripper at a location in the isostripper at which the material within that portion of the isostripper most closely approximates the composition of said bottom product stream.

References Cited UNITED STATES PATENTS 3,253,054 5/1966 Van Pool 260-68348 3,211,536 10/1965 Van Pool 260--683.48

DELBERT E. GANTZ, Primary Examiner G. I. CRASANAKIS, Assistant Examiner US. Cl. X.R. 208354 

